The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a printer, or a facsimile device, and in particular, to an image forming apparatus wherein a toner image on an image carrier is transferred onto an intermediate transfer belt as a primary transfer, and then, the toner image is transferred onto a transfer material as a secondary transfer.
In the conventional primary transfer means wherein an intermediate transfer belt is brought into contact with an image carrier, and the intermediate transfer belt is pressed from its inside by a transfer means such as a transfer roller for transferring, if primary transfer roller 25 is at the position that is exactly the left of photoreceptor 10 as shown in FIG. 2(a), a vibration from side to side is caused on primary transfer roller 25 by rotations of the photoreceptor 10 and of the intermediate transfer belt 20. Thus, this vibration causes transfer unevenness easily, and causes a problem that an electric field is formed between the intermediate transfer belt 20 and photoreceptor 10 at vicinity R where the intermediate transfer belt 20 starts making contact with photoreceptor 10, and this electric field causes toner on the photoreceptor 10 to fly in the direction of arrow t, resulting in the so-called dust phenomenon of a toner image. To solve this problem, there has been taken a structure to provide a pressure-contact member which brings a photoreceptor into contact with an intermediate transfer belt more closely, on the upstream side of a transfer means, or there has been taken a measure to shift primary transfer roller 25 to the downstream side by a certain amount as shown with dotted lines, for reducing an effect of an electric field at vicinity R where the intermediate transfer belt starts touching photoreceptor 10 (Japanese TOKKAIHEI No. 9-152791).
However, despite the aforementioned technology, there are some cases where transfer efficiency cannot be secured sufficiently because of vibration of the primary transfer roller caused by fluctuation of tension of the intermediate transfer belt, deviation of the image carrier and waviness of the intermediate transfer belt.
An object of the invention is to provide an image forming apparatus wherein effects of vibration of the primary transfer roller, deviation of the image carrier and of waviness of the intermediate transfer belt are avoided, dust phenomenon of toner images for characters and lines and transfer unevenness are not caused, and stable and excellent transfer efficiency can be kept.
An image forming apparatus having therein an image carrier, an intermediate transfer belt that is trained about plural rollers and is located at a position where it comes in contact with the image carrier, a primary transfer roller that is arranged to face the image carrier through the intermediate belt for the purpose of transferring toner on the image carrier onto the intermediate transfer belt, wherein the following expression holds when Ft (kgf) represents tension of the intermediate transfer belt, Fr (kgf) represents pressing force of primary transfer roller and d (mm) represents a shift amount for the image carrier and the primary transfer roller,Fr≧(0.05×Ft+0.11)×d−0.3.